Temporal.Calendar

Table of Contents

A Temporal.Calendar is a representation of a calendar system. It includes information about how many days are in each year, how many months are in each year, how many days are in each month, and how to do arithmetic in that calendar system.

Much of the world uses the Gregorian calendar, which was invented in 1582 C.E. On the modern Internet, the most often used calendar system is the calendar standardized by ISO 8601, which is the same as the Gregorian calendar with the addition of week-numbering rules. In general it is extended backwards ("proleptically") to cover the period of history before its invention, which is an optional modification allowed by the ISO 8601 standard.

However, the ISO 8601 calendar is not the only calendar in common use in the world. Some places use another calendar system as the main calendar, or have a separate calendar system as a commonly-used civil or religious calendar.

When to use Temporal.Calendar

It is best practice to specify a calendar system when performing calendar-sensitive operations, which are those involving arithmetic or other calculation in months or years.

For example, to add a month to a date in the Hebrew calendar:

date.withCalendar('hebrew').add({ months: 1 });

Temporal types' toLocaleString() methods use the user's preferred calendar, without needing to call withCalendar(). To perform arithmetic consistently with the toLocaleString() calendar system:

const calendar = new Intl.DateTimeFormat().resolvedOptions().calendar;
date.withCalendar(calendar).add({ months: 1 });

Invariants Across Calendars

The following "invariants" (statements that are always true) hold for all built-in calendars, and should also hold for any properly-authored custom calendar that supports years, months, and days units:

Writing Cross-Calendar Code

Here are best practices for writing code that will work regardless of the calendar used:

Custom calendars

For specialized applications where you need to do calculations in a calendar system that is not supported by Intl, you can implement a custom calendar. There are two ways to do this.

The recommended way is to create a class inheriting from Temporal.Calendar. You must use one of the built-in calendars as the "base calendar". In the class's constructor, call super() with the identifier of a built-in calendar to serve as a base. The class must override the id prototype property, and should also override toString() and toJSON() to match. Overriding all the other properties of Temporal.Calendar.prototype is optional. Any property that's not overridden will behave as in the base calendar. It's recommended to override dateFromFields(), monthDayFromFields(), yearMonthFromFields(), and dateAdd()so that they return Temporal objects with the custom calendar and not the base calendar.

The other, more difficult, way to create a custom calendar is to create a plain object implementing the Temporal.Calendar protocol, without subclassing. The object must implement all of the Temporal.Calendar properties and methods except for toString() and toJSON(). Any object with the required properties and methods will return the correct output from any Temporal property or method. However, most other code will assume that custom calendars act like built-in Temporal.Calendar objects. To interoperate with libraries or other code that you didn't write, then you should implement the toString() and toJSON() methods as well.

The identifier of a custom calendar must consist of one or more components of between 3 and 8 ASCII alphanumeric characters each, separated by dashes, as described in Unicode Technical Standard 35.

Custom calendars are responsible for interpreting and validating all inputs, including options. Calendars should (and built-in calendars will) throw a TypeError if a required option is missing or has the wrong type, but throw a RangeError if it's present but has an invalid value.

Calendars are also responsible for assigning default values. For example, if the overflow option is undefined, it will be interpreted by built-in calendars as 'constrain'. Custom calendars should maintain this behavior unless there's a good reason not to. Calendars can also accept additional non-default values for existing options or can accept new options that built-in calendars don't. When adding new options, calendar authors should use a unique prefix, e.g. the name of the calendar, to avoid potential conflicts with future options which may be used by Temporal.

Handling unusual dates: leap days, leap months, and skipped or repeated periods

Calendars can vary from year to year. Solar calendars like 'gregory' use leap days. Lunar calendars like 'islamic' adjust month lengths to lunar cycles. Lunisolar calendars like 'hebrew' or 'chinese' have "leap months": extra months added every few years.

Calendars may also have one-time changes. The built-in 'gregory' calendar in ECMAScript doesn't skip days because it's a proleptic Gregorian calendar, but other calendars may skip days, months, or even years. For example, a non-proleptic custom calendar for France would have 4 October 1582 (the last day of the Julian calendar) directly followed by 15 October 1582 (the first day of the Gregorian calendar), skipping 10 calendar days.

Calendar variation across years means that programs may encounter historical dates that are valid in one year but invalid in another. A common example is calling toPlainDate on a Temporal.PlainMonthDay object to convert a birthday or anniversary that originally fell on a leap day, leap month, or other skipped period. Temporal types' with or from methods can run into the same issue.

When Temporal encounters inputs representing a month and/or day that doesn't exist in the desired calendar year, by default (overridable in with or from via the overflow option) the inputs will be adjusted using the following algorithm:

Authors of custom calendars (especially Julian-to-Gregorian calendars that include a skipped period) must follow the rules above; otherwise, callers may experience unexpected behavior.

Finally, just like calendars can sometimes skip days or months, it is possible for real-world calendars to repeat dates, for example when a country transitions from one calendar system to another. No current built-in calendar repeats dates, but a custom calendar that includes repeated dates may add custom fields and/or options to help users handle this case.

Constructor

new Temporal.Calendar(calendarIdentifier: string) : Temporal.Calendar

Parameters:

Returns: a new Temporal.Calendar object.

For a list of calendar identifiers, see the documentation for Intl.DateTimeFormat. If calendarIdentifier is not a built-in calendar, then a RangeError is thrown.

Use this constructor directly if you have a string that is known to be a correct built-in calendar identifier. If you have an ISO 8601 date-time string with a [u-ca=identifier] annotation, then Temporal.Calendar.from() is more convenient than parsing the identifier out of the string.

Example usage:

cal = new Temporal.Calendar('iso8601');
cal = new Temporal.Calendar('gregory');
/* WRONG */ cal = new Temporal.Calendar('discordian'); // => throws, not a built-in calendar

Static methods

Temporal.Calendar.from(thing: any) : Temporal.Calendar

Parameters:

Returns: a calendar object.

This static method creates a new calendar from another value. If the value is another Temporal.Calendar object, or object implementing the calendar protocol, the same object is returned. If the value is another Temporal object that carries a calendar or an object with a calendar property, such as a Temporal.ZonedDateTime, the object's calendar is returned.

Any other value is required to be a string that is either:

Note that the ISO 8601 string can be extended with a [u-ca=identifier] annotation in square brackets appended to it. Without such an annotation, the calendar is taken to be iso8601.

This function is often more convenient to use than new Temporal.Calendar() because it handles a wider range of input.

Usage examples:

// Calendar names
cal = Temporal.Calendar.from('iso8601');
cal = Temporal.Calendar.from('gregory');

// ISO 8601 string with or without calendar annotation
cal = Temporal.Calendar.from('2020-01-13T16:31:00.065858086');
cal = Temporal.Calendar.from('2020-01-13T16:31:00.065858086-08:00[America/Vancouver][u-ca=iso8601]');
/* WRONG */ cal = Temporal.Calendar.from('[u-ca-iso8601]'); // => throws, lone annotation not a valid ISO 8601 string

// Existing calendar object
cal2 = Temporal.Calendar.from(cal);

// Custom calendar names cannot be parsed from strings
/* WRONG */ cal = Temporal.Calendar.from('discordian'); // => throws, not a built-in calendar

Properties

calendar.id : string

The id property gives an unambiguous identifier for the calendar. Effectively, this is whatever calendarIdentifier was passed as a parameter to the constructor.

When subclassing Temporal.Calendar, this property must be overridden to provide an identifier for the custom calendar.

Methods

calendar.era(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string) : string | undefined

calendar.eraYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string) : number | undefined

calendar.year(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string) : number

calendar.month(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string) : number

calendar.monthCode(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | Temporal.PlainMonthDay | object | string) : string

calendar.day(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainMonthDay | object | string) : number

calendar.dayOfWeek(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | object | string): number

calendar.dayOfYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | object | string): number

calendar.weekOfYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | object | string): number

calendar.yearOfWeek(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | object | string): number

calendar.daysInWeek(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | object | string): number

calendar.daysInMonth(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string): number

calendar.daysInYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string): number

calendar.monthsInYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string): number

calendar.inLeapYear(date: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime | Temporal.PlainYearMonth | object | string): boolean

The above methods are all similar. They provide a way to query properties of a particular date in the calendar's date reckoning.

Parameters:

Returns: some piece of data (year, month, day, etc., depending on the method) associated with date, in calendar's calendar system.

If date is not one of the appropriate Temporal objects, then it will be converted to a Temporal.PlainDate as if it were passed to Temporal.PlainDate.from().

None of the above methods need to be called directly except in specialized code. They are called indirectly when reading the various properties of Temporal.ZonedDateTime, Temporal.PlainDateTime, Temporal.PlainDate, Temporal.PlainMonthDay, or Temporal.PlainYearMonth.

For example:

const date = Temporal.PlainDate.from('2019-02-06').withCalendar('hebrew');
date.year; // => 5779
date.calendar.year(date); // same result, but calling the method directly
date.monthCode; // => 'M05L'
date.calendar.monthCode(date); // same result, but calling the method directly
date.daysInYear; // => 385
date.calendar.daysInYear(date); // same result, but calling the method directly

calendar.dateFromFields(fields: object, options: object) : Temporal.PlainDate

calendar.yearMonthFromFields(fields: object, options: object) : Temporal.PlainYearMonth

calendar.monthDayFromFields(fields: object, options: object) : Temporal.PlainMonthDay

The above three methods are similar. They provide a way to construct other Temporal objects from values in the calendar's date or time reckoning.

Parameters:

Returns: a new Temporal.PlainDate, Temporal.PlainYearMonth, or Temporal.PlainMonthDay object, respectively.

None of the above methods need to be called directly except in specialized code. They are called indirectly when using Temporal.PlainDate.from(), Temporal.PlainDateTime.from(), Temporal.PlainYearMonth.from(), and Temporal.PlainMonthDay.from().

A custom implementation of these methods would convert the calendar-space arguments to the ISO 8601 calendar, and return an object created using new Temporal.PlainDate(...isoArgs), with PlainYearMonth and PlainMonthDay substituted for PlainDate as appropriate.

For example:

date = Temporal.PlainDate.from({ year: 5779, monthCode: 'M05L', day: 18, calendar: 'hebrew' });
date.year; // => 5779
date.month; // => 6
date.monthCode; // => 'M05L'
date.day; // => 18
date.toString(); // => '2019-02-23[u-ca=hebrew]'
date.toLocaleString('en-US', { calendar: 'hebrew' }); // => '18 Adar I 5779'

// same result, but calling the method directly and using month index instead of month code:
date = Temporal.Calendar.from('hebrew').dateFromFields(
  { year: 5779, month: 6, day: 18 },
  { overflow: 'constrain' }
);

calendar.dateAdd(date: Temporal.PlainDate | object | string, duration: Temporal.Duration | object | string, options: object) : Temporal.PlainDate

This method provides a way to do time arithmetic in the calendar's date reckoning.

Parameters:

Returns: a new Temporal.PlainDate object.

If date is not a Temporal.PlainDate object, or duration not a Temporal.Duration object, then they will be converted to one as if they were passed to Temporal.PlainDate.from() or Temporal.Duration.from(), respectively.

This method does not need to be called directly except in specialized code. It is called indirectly when using add() and subtract() of Temporal.PlainDateTime, Temporal.PlainDate, and Temporal.PlainYearMonth.

A custom implementation of this method would perform the calendar-specific addition, convert the result to the ISO 8601 calendar, and return an object created using new Temporal.PlainDate(...isoArgs).

For example:

date = Temporal.PlainDate.from('2020-05-29')
  .withCalendar('islamic')
  .add(Temporal.Duration.from({ months: 1 }), { overflow: 'reject' });
date.year; // => 1441
date.month; // => 11
date.day; // => 7
date.toString(); // => '2020-06-28[u-ca=islamic]'

// same result, but calling the method directly:
date = Temporal.Calendar.from('islamic').dateAdd(
  Temporal.PlainDate.from('2020-05-29'),
  Temporal.Duration.from({ months: 1 }),
  { overflow: 'reject' }
);
date.year; // => 1441
date.month; // => 11
date.day; // => 7
date.toString(); // => '2020-06-28[u-ca=islamic]'

calendar.dateUntil(one: Temporal.PlainDate | object | string, two: Temporal.PlainDate | object | string, options: object) : Temporal.Duration

Parameters:

Returns: a Temporal.Duration representing the time elapsed after one and until two.

If either of one or two are not Temporal.PlainDate objects, then they will be converted to one as if they were passed to Temporal.PlainDate.from().

This method does not need to be called directly except in specialized code. It is called indirectly when using the until() and since() methods of Temporal.PlainDateTime, Temporal.PlainDate, Temporal.PlainYearMonth, and Temporal.ZonedDateTime.

If one is later than two, then the resulting duration should be negative.

The default largestUnit value of 'auto' is the same as 'day'.

NOTE: Unlike Temporal.Calendar.dateAdd(), the options object that this method receives is not always the same object passed to the respective until() or since() method. Depending on the type, a copy may be made of the object.

For example:

d1 = Temporal.PlainDate.from('2020-07-29').withCalendar('chinese');
d2 = Temporal.PlainDate.from('2020-08-29').withCalendar('chinese');
d1.until(d2, { largestUnit: 'month' }); // => P1M2D

// same result, but calling the method directly:
Temporal.Calendar.from('chinese').dateUntil(
  Temporal.PlainDate.from('2020-07-29'),
  Temporal.PlainDate.from('2020-08-29'),
  { largestUnit: 'month' }
); // => P1M2D

calendar.fields(fields: Iterable<string>) : string[]

Parameters:

Returns: a new array of field names.

This method does not need to be called directly except in specialized code. It is called indirectly when using the from() static methods and with() methods of Temporal.PlainDateTime, Temporal.PlainDate, Temporal.PlainMonthDay, Temporal.PlainYearMonth, and Temporal.ZonedDateTime, and a number of other methods.

Custom calendars should override this method if they accept fields in from() or with() other than the standard set of built-in calendar fields: year, month, monthCode, and day. The input array contains the field names that are necessary for a particular operation (for example, 'monthCode' and 'day' for Temporal.PlainMonthDay.prototype.with()). The method should make a copy of the array and add additional fields as needed.

When subclassing Temporal.Calendar, this method doesn't need to be overridden, unless your calendar requires extra fields, because the default implementation returns a copy of fields.

Usage example:

// In the ISO 8601 calendar, this method just makes a copy of the input array
Temporal.Calendar.from('iso8601').fields(['monthCode', 'day']);
// => [ 'monthCode', 'day' ]

calendar.mergeFields(fields: object, additionalFields: object) : object

Parameters:

Returns: a new object with properties from both fields and additionalFields.

This method does not need to be called directly except in specialized code. It is called indirectly when using the with() methods of Temporal.PlainDateTime, Temporal.PlainDate, Temporal.PlainMonthDay, Temporal.PlainYearMonth, and Temporal.ZonedDateTime.

Custom calendars should override this method if they allow a calendar unit to be specified in more than one way. (For example, the Gregorian calendar allows years to be specified either by a year property or a combination of era and eraYear.) The overridden implementation should return an object with some or all of the properties from the original fields object and additionalFields copied onto it.

When subclassing Temporal.Calendar, this method doesn't need to be overridden, unless your calendar adds more ways to specify a unit other than the built-in properties monthCode, era, and eraYear. The default implementation copies all properties from additionalFields onto fields, taking into account that months may be specified either by month or monthCode properties, and any other special cases required by built-in calendars.

Usage example:

// In built-in calendars, this method copies properties, taking `month`
// and `monthCode` into account
Temporal.Calendar.from('iso8601').mergeFields(
  { year: 2006, month: 7, day: 31 },
  { monthCode: 'M08' }
);
// => { year: 2006, monthCode: 'M08', day: 31 }

calendar.toString() : string

Returns: The string given by calendar.id.

This method overrides Object.prototype.toString() and provides the calendar's id property as a human-readable description.

Example usage:

Temporal.PlainDate.from('2020-05-29[u-ca=gregory]').calendar.toString(); // => 'gregory'

calendar.toJSON() : string

Returns: The string given by calendar.id.

This method is the same as calendar.toString(). It is usually not called directly, but it can be called automatically by JSON.stringify().

The reverse operation, recovering a Temporal.Calendar object from a string, is Temporal.Calendar.from(), but it cannot be called automatically by JSON.parse(). If you need to rebuild a Temporal.Calendar object from a JSON string, then you need to know the names of the keys that should be interpreted as Temporal.Calendars. In that case you can build a custom "reviver" function for your use case.

When subclassing Temporal.Calendar, this method doesn't need to be overridden because the default implementation returns the result of calling calendar.toString().

Example usage:

const user = {
  id: 775,
  username: 'robotcat',
  password: 'hunter2', // Note: Don't really store passwords like that
  userCalendar: Temporal.Calendar.from('gregory')
};
const str = JSON.stringify(user, null, 2);
console.log(str);
// =>
// {
//   "id": 775,
//   "username": "robotcat",
//   "password": "hunter2",
//   "userCalendar": "gregory"
// }

// To rebuild from the string:
function reviver(key, value) {
  if (key.endsWith('Calendar')) return Temporal.Calendar.from(value);
  return value;
}
JSON.parse(str, reviver);